1. Field of the Invention
The present invention relates to a high-frequency switch module to switch between transmission and reception of a communication signal having a specific frequency, and more particularly, the present invention relates to a high-frequency switch module using a FET (field-effect transistor) switch.
2. Description of the Related Art
At present, a radio communication method for mobile phones or other devices has a plurality of specifications. For example, a multi-band GSM (global system for mobile communications) method has been adopted in Europe. In the GSM method, a plurality of communication signals (transmission/reception signals) having different frequency bands, e.g., an 850 MHz band and a 900 MHz band, are used. A 1800 MHz band and a 1900 MHz band are also used. When such a plurality of communication signals having different frequency bands are transmitted/received by one antenna, communication signals other than a communication signal of a desired frequency band are not required. Furthermore, a reception signal is not required during transmission, whereas a transmission signal is not required during reception. Therefore, in order to perform transmission and reception with one antenna, the antenna must switch between a path to transmit a transmission signal of a desired communication signal and a path to transmit a reception signal of a desired communication signal. Various types of high-frequency switch modules including a FET switch have been used (e.g., see Japanese Unexamined Patent Application Publication No. 2002-185356 (Patent Document 1)).
Patent Document 1 discloses a high-frequency switch module shown in FIG. 7.
FIG. 7 is a block diagram showing a configuration of the known high-frequency switch module.
The known high-frequency switch module includes a FET switch SW100, which includes a transmission port RF101 to receive a transmission signal of a first communication signal (first transmission signal) and a transmission signal of a second communication signal (second transmission signal), a first reception port RF102 to output a reception signal of the first communication signal (first reception signal), a second reception port RF103 to output a reception signal of the second communication signal (second reception signal), and an antenna port ANTO to output the first and second transmission signals to an antenna and to receive the first and second reception signals from the antenna. A switch made of a semiconductor, particularly a FET, is used as the FET switch SW100. A GeAs switch is used in many cases. In this known high-frequency switch module, a low-pass filter LPF201 to attenuate harmonics of the first and second transmission signals is connected to the transmission port RF101, a band-pass filter BPF301 to allow a fundamental of the first reception signal to pass therethrough is connected to the first reception port RF102, and a band-pass filter BPF302 to allow a fundamental of the second reception signal to pass therethrough is connected to the second reception port RF202.
In the above-described high-frequency switch module, a transmission signal is input to a transmission signal input terminal TX1 connected to the transmission port RF101 of the FET switch SW100 through the low-pass filter LPF201. The transmission signal is input after being amplified by a power amplifier PA that is typically connected in a previous stage. At the amplification, higher-order harmonics to a fundamental frequency fo of the transmission signal are generated and are input together with the transmission signal of the fundamental frequency fo. Herein, if the low-pass filter LPF201 of the high-frequency switch module shown in FIG. 7 is set so as to attenuate the higher-order harmonics, the higher-order harmonics of the transmission signal input to the FET switch SW100 can be suppressed. For example, if the low-pass filter LPF201 includes a low-pass filter to attenuate a second-order harmonic of the fundamental frequency fo (2·fo) and a low-pass filter to attenuate a third-order harmonic of the fundamental frequency fo (3·fo), the second-order and third-order harmonics are suppressed.
However, if the FET switch SW100 is a GaAs switch and if a high-frequency transmission signal is input thereto, harmonic distortion occurs in the FET switch SW100, and thus, harmonics including a double harmonic and a triple harmonic are evenly output to the respective ports. At this time, when the low-pass filter LPF201 is viewed from the transmission port RF101 in the frequency of the above-described harmonics, the impedance value approaches infinity, this means open state, and the harmonics generated in the FET switch SW100 are completely reflected at the transmission port RF101 side end of the low-pass filter LPF201 and are input to the FET switch SW100. As a result, assuming that the original harmonics are “X” and that the additional harmonics caused by the complete reflection are “α”, harmonics “X+α” are output from the antenna port ANT0.
Such harmonics may be suppressed by using a GaAs switch that suppresses generation of harmonics. However, such a switch does not actually exist. Also, generation of harmonics may be suppressed by using a switch circuit including a diode switch. However, at least two diodes are required for switching between transmission and reception of each communication signal and a circuit added to those diodes is also necessary, such that the high-frequency switch module cannot be miniaturized. Furthermore, use of a plurality of diode switches causes an increase in power consumption and a decrease in response speed. Particularly, these disadvantages become significant as the number of ports of the FET switch increases.